To date, electric airplane design efforts have focused on replacing small internal combustion engines on motor gliders and ultra light airplanes and with a single electric motor of near equal power. However, electric propulsion offers an opportunity to redesign the aircraft itself and distribute the power about the airframe synergistically enhancing the aerodynamics. Where internal combustion engines are complex and need to be large for efficiency, electric motors are simple and many small ones can be used. The question is; how to make the best use of this distributed potential? This concept directly addresses this question and adds capabilities not possible with a single or a small number of internal combustion engines or electric motors.
This concept builds on three technologies: 1) electric ducted fans, 2) upper surface flow enhancement and 3) distributed propulsion of aircraft. These are combined in a novel way to achieve benefits not foreseen by those separately; specifically, improved aerodynamics and control of span-wise lift distribution. The following is a brief description of the relevant technologies.
A ducted fan is a propulsion system where a mechanical fan, which is a type of propeller, is mounted within a cylindrical shroud or duct.
In recent years, electrically powered ducted fans (EDFs) have been developed for radio controlled model airplanes. They have become commonly available ranging in size from 30 mm to 120 mm, with power up to 15 hp (11 kw). The efficiency of these units has increased over time and continues to evolve with both motor and rotor improvements.
In 2014 Airbus announced its development of the E-Fan, a single passenger airplane, powered by two electrically powered ducted fans mounted on the sides of the fuselage aft of the wing. According to Airbus, EDFs offer many advantages. The first is higher efficiency than an open propeller below 100-110 mph, with 80% propulsion efficiency (percent of delivered mechanical power that is converted to thrust). Another advantage is smaller size than a comparable propeller, while noise is reduced. Additionally, EDFs offer protection of ground personnel when the engine is running.
Most of the lift generated by an airfoil is caused by an increase in the flow velocity over the upper surface. Willard Custer explored enhancing the lift by wrapping the wing around the lower half of the propeller arc with his “Custer Channel Wing” aircraft. He was able to show improved short take-off and landing (STOL) capabilities with his aircraft.
Where Custer used the propeller to accelerate the air over the upper surface of his “channels”, another approach is to use a “blown upper surface”, a wing with air or other gas released through a slot in the upper surface generally in the direction of airflow to enhance the lift characteristics of the wing while providing thrust. There is a long history of blown upper surface technologies leading through the Bartoe/Bell-Jetwing and culminating in the 1980s with the development of the Boeing YC-14.
The Bartoe/Bell-Jetwing featured a single jet engine mounted in the fuselage with its exhaust ducted to 70% of the trailing edge of the wings providing thrust and augmenting the lift by increasing the air velocity over the wing. This jet powered aircraft could fly at 350 mph and yet remained controllable to airspeeds as low as 40 mph, landing in less than 300 ft.
The Boeing YC-14 featured two jet engines, one over each side of a high wing, positioned close to the fuselage blowing their exhaust over a small span of the upper surface. The Jetwing, the YC-14, and similar experimental aircraft increase air velocity over a portion of the top surface of the wing to not only improve the airfoil's lift curve slope, but raising the maximum lift coefficient. Additionally, the use of flaps on the trailing edge of the wing with the blown air flow going over them, further enhances the lift. Using these techniques, the YC-14 was able to provide exceptional STOL performance.
Distributed propulsion is the integration of the airflows and forces generated by the propulsion system over a large portion of the aircraft in such a way as to improve the vehicle's aerodynamics, and propulsive and structural efficiencies. Historically, gains in aircraft performance through distributed propulsion were outweighed by complexity when using traditional power plants.
The Bartoe/Bell Jetwing had distributed propulsion as did the Hunting H.126 flown in the 1960's. It diverted almost 60% of its jet engine thrust across its wing's trailing edge to achieve lift coefficients up to a theoretical 7.5 and an operational 5.5, far above that possible without the jet flap. The aircraft was an experimental platform and not a practical vehicle and was abandoned.
A resurgence in interest in distributed propulsion has been fueled by the potential for many small electric motors as a replacement for a few large internal combustion or jet engines; and advances in computational and experimental tools along with new technologies in materials, structures, and aircraft controls, etc. enabling a high degree of integration of the airframe and propulsion system in aircraft design. This integration allows the potential of synergistic coupling of airframe aerodynamics and the propulsion by distributing thrust using many propulsors on the airframe to drastically reduce aircraft related fuel burn, emissions, and noise.
Most recently, distributed propulsion has focused on two areas. Firstly; NASA, DARPA and many contractors have led the study of distributed propulsion on large, high speed airplanes for use as airliners and cargo transport. Secondly, NASA is leading an effort aimed at small, low speed business, passenger, and general aviation aircraft. The large passenger and transport configurations have tended toward blended-wing body configurations with the distributed propulsors arranged on the upper rear of the center section of the airplane to ingest the boundary layer.
A smaller aircraft is the LEAPTech (funded by NASA), now called “Maxwell” or the X-57. It is scheduled for flight testing spring 2018. This aircraft has 14 small electric motors on the leading edge of the wing each powering a propeller. Each propeller can fold when its motor is shut down. In this way, all the motors are propelling the airplane on takeoff and some motors shut down for cruise. Using the distributed propulsion with the increased air velocity over its wing, the X-57 will have a much smaller wing than a similar airplane using a single engine. This leads to lower induced and parasite drag resulting in higher flight efficiency—increased speed and range on less energy.
A limitation of the X-57 configuration is that the integration of propulsion with aerodynamics is purely in terms of increasing the speed of the air over both the top and bottom of the wing. This does not take full advantage of the blown upper surface potential. Additionally, due to propeller swirl, the angle of attack of air flowing over wing is not uniform over the wing span. In fact, the wing sees three different flow fields: 1) span not affected by a propeller, 2) span affected by the downward motion of the propeller, and 3) span affected by upward motion of the propeller. The resulting uneven angle of attack over the span of the wing means that not all sections are flying with the same efficiency in terms of the lift and drag produced. A final limitation is that a majority of the propulsors are shut down during cruise thus nullifying any potential for propulsion-aerodynamic integration during a majority of the flight envelope.